Spending most of your day in the murky depths of a river sucking in water would be enough to make anyone feel blue.

Despite its name, the depressed river mussel is a success in Sussex, thriving in record numbers in the River Arun when it is dwindling elsewhere.

Now scientists from overseas are about to get to the bottom of the riverbed world of the extremely-rare mollusc.

The scientists from America, Canada and Cambridge University have swooped on the River Arun to find out why it is such an ideal habitat for the mussels.

They donned scuba gear and dived to the bottom of the Arun at Greatham Bridge, near Watersfield, Arundel, to dig up the mussels.

Pollution and loss of suitable rivers means the population elsewhere in the country is in decline.

The depressed, or compressed mussel, so-called because of the relative thinness of its shell, normally accounts for one in 50 of the mussels in British rivers.

This spot in the river was first documented as being rich in mussels by the Roman and here, one in five of all mussels is depressed.

The mussels, which live for up to 15 years, cannot be eaten because of the large amount of organic waste they filter through their system.

If people were too eat one of these mussels, the chances are they would be violently ill for the next 24 hours.

The Environment Agency has given £1,500 to support the project and it is hoped the river studies will help scientists in Canada and the USA stem the decline of freshwater mussels worldwide.

Paul Smith, conservation and recreation officer at the Environment Agency, said: "This is probably one of the best places in the world for these mussels. It is a really exciting site.

Phil Griffiths, conservation and recreation team leader, said: "We are delighted to be able to help ensure the mussel population throughout the world is protected and, where possible, enhanced.

"Freshwater mussels are good indicators of clean water. They are also very important to other organisms in the river.

"They act as natural water filters because they remove a large amount of the suspended particles from the water. Healthy mussel populations lead to lots of water plants, insects and fish."

Dr David Aldridge, a researcher from Cambridge University, said the mussels were good indicators of clean water.

He added: "This is possibly the best place in the world for these mussels.

"We are very lucky and the good thing about the River Arun is that, because of the high number of mussels, we can find out what part of the river they prefer living in."